Entries from May 2012 ↓

Resonant Antennas

We often hear hams say that a resonant antenna is the best antenna. Why? Because they work. Here are the reasons why they work.

To be resonant an antenna has to be purely resistive at its design frequency. That resistve value is somewhere arouned 50 to 75 ohms. That is a very convenient match to coaxial cable and also a match to most transmitters. It is this combination of conditions that allow resonant antennas to work as well as they do because these conditions allow maximum power transfer to the antenna with minimum loss. It is not caused by some sort of magic hokus-pokus associated with the fact that the antenna is resonant. For instance, if you changed out the coax from 50-75 ohm to 200 ohm, the antenna would still be resonant but it would not work nearly as well because there would be excessive loss in the mismatched coax. That mismatch would also cause the transmitter to see a higher SWR which would fold back the amount of power the transmitter produces. As a result you would be starting out with less power to lose in the first place.

We can overcome the foldback situation by adding an antenna tuner or transmatch at the transmitter output. This will allow the transmitter to see a perfect match but we still have a missmatch in the coax and possibly at the antenna.

We could ignore the loss in the transmission line due to missmatch if we use open wire line for the transmission line. Doing so will not correct the mismatch but it will reduce the loss to become nearly negligible. The antenna tuner will also allow fairly efficient use of the antenna on frequencies where it is not resonant.

Not everyone has a need for this type of multiband antenna or a desire to buy an antenna tuner. Those folk might prefer to stay with the original resonant antenna and use the proper coax it requires. That saves the cost of an antenna tuner and reduces the number of knobs that need tweaking to effect a band change. At first glance this solution also creates a single band antenna.

We can multi-band an antenna by adding traps or by adding multiple legs to the dipole. Both methods do not require an antenna tuner and can be implemented using standard low impedance coax.

The method using the fan dipole approach with multiple dipole legs, two per band, is preferable to using traps. Traps have losses and are prone to fail at high power levels. More importantly, traps reduce the bandwidth available. Thus you may not be able to cover the CW and SSB portions of a band with one antenna. You may not be able to do that anyway even without traps depending on the band being considered but traps will make it more difficult.

While it is true that resonant antennas do work well, There is no reason why non-resonant antennas can work just as well. The secret to making them work lies in knowing what it takes.

Then isn’t that true about most things. The secret to success nearly always depends on a person knowing what they are doing.